This invention relates to a circuit arrangement for a photometer with a photocell, which is alternately shielded and exposed to light incidence by a darkening device and whose signal is amplified by an amplifier connected to a logarithmic amplifier. U.S. Pat. No. 3,892,493 describes a photometric process, and a photometer operating in accordance with this process, wherein a compensation takes place of the errors occurring in a photocell and in a measuring signal amplifier. For this purpose, the photocell is screened in an alternating sequence by a darkening device from the incidence of light and then freed. The signal occurring in the darkened phase being composed of pure error constituents which are stored. The stored signal is placed in opposition to the measuring signal in the actual measuring phase during which the photocell is freed and exposed to the measuring light, so that the measuring light is free of error constituents. In an exemplary embodiment described in such patent, this measuring signal is fed to a logarithmic amplifier, and in this manner a logarithmic indication is obtained, as it is needed in numerous applications in photographic technology. Such logarithmic amplifiers generally have a very good response with reference to their transmission function. Should light of very low intensity factors be measured, however, the errors primarily emanating from the input voltage drift of the logarithmic amplifier are so great in proportion to the measuring signal that measurements with a circuit arrangement according to U.S. Patent 3,892,493 can no longer be carried out with sufficient accuracy.
The object of the invention is to provide a circuit arrangement for a photometer of the aforementioned type wherein the errors occurring due to the logarithmic amplifier exert no influence on the measuring results.